You can not select more than 25 topics
Topics must start with a letter or number, can include dashes ('-') and can be up to 35 characters long.
549 lines
21 KiB
549 lines
21 KiB
Packfile transfer protocols |
|
=========================== |
|
|
|
Git supports transferring data in packfiles over the ssh://, git:// and |
|
file:// transports. There exist two sets of protocols, one for pushing |
|
data from a client to a server and another for fetching data from a |
|
server to a client. All three transports (ssh, git, file) use the same |
|
protocol to transfer data. |
|
|
|
The processes invoked in the canonical Git implementation are 'upload-pack' |
|
on the server side and 'fetch-pack' on the client side for fetching data; |
|
then 'receive-pack' on the server and 'send-pack' on the client for pushing |
|
data. The protocol functions to have a server tell a client what is |
|
currently on the server, then for the two to negotiate the smallest amount |
|
of data to send in order to fully update one or the other. |
|
|
|
Transports |
|
---------- |
|
There are three transports over which the packfile protocol is |
|
initiated. The Git transport is a simple, unauthenticated server that |
|
takes the command (almost always 'upload-pack', though Git |
|
servers can be configured to be globally writable, in which 'receive- |
|
pack' initiation is also allowed) with which the client wishes to |
|
communicate and executes it and connects it to the requesting |
|
process. |
|
|
|
In the SSH transport, the client just runs the 'upload-pack' |
|
or 'receive-pack' process on the server over the SSH protocol and then |
|
communicates with that invoked process over the SSH connection. |
|
|
|
The file:// transport runs the 'upload-pack' or 'receive-pack' |
|
process locally and communicates with it over a pipe. |
|
|
|
Git Transport |
|
------------- |
|
|
|
The Git transport starts off by sending the command and repository |
|
on the wire using the pkt-line format, followed by a NUL byte and a |
|
hostname parameter, terminated by a NUL byte. |
|
|
|
0032git-upload-pack /project.git\0host=myserver.com\0 |
|
|
|
-- |
|
git-proto-request = request-command SP pathname NUL [ host-parameter NUL ] |
|
request-command = "git-upload-pack" / "git-receive-pack" / |
|
"git-upload-archive" ; case sensitive |
|
pathname = *( %x01-ff ) ; exclude NUL |
|
host-parameter = "host=" hostname [ ":" port ] |
|
-- |
|
|
|
Only host-parameter is allowed in the git-proto-request. Clients |
|
MUST NOT attempt to send additional parameters. It is used for the |
|
git-daemon name based virtual hosting. See --interpolated-path |
|
option to git daemon, with the %H/%CH format characters. |
|
|
|
Basically what the Git client is doing to connect to an 'upload-pack' |
|
process on the server side over the Git protocol is this: |
|
|
|
$ echo -e -n \ |
|
"0039git-upload-pack /schacon/gitbook.git\0host=example.com\0" | |
|
nc -v example.com 9418 |
|
|
|
If the server refuses the request for some reasons, it could abort |
|
gracefully with an error message. |
|
|
|
---- |
|
error-line = PKT-LINE("ERR" SP explanation-text) |
|
---- |
|
|
|
|
|
SSH Transport |
|
------------- |
|
|
|
Initiating the upload-pack or receive-pack processes over SSH is |
|
executing the binary on the server via SSH remote execution. |
|
It is basically equivalent to running this: |
|
|
|
$ ssh git.example.com "git-upload-pack '/project.git'" |
|
|
|
For a server to support Git pushing and pulling for a given user over |
|
SSH, that user needs to be able to execute one or both of those |
|
commands via the SSH shell that they are provided on login. On some |
|
systems, that shell access is limited to only being able to run those |
|
two commands, or even just one of them. |
|
|
|
In an ssh:// format URI, it's absolute in the URI, so the '/' after |
|
the host name (or port number) is sent as an argument, which is then |
|
read by the remote git-upload-pack exactly as is, so it's effectively |
|
an absolute path in the remote filesystem. |
|
|
|
git clone ssh://user@example.com/project.git |
|
| |
|
v |
|
ssh user@example.com "git-upload-pack '/project.git'" |
|
|
|
In a "user@host:path" format URI, its relative to the user's home |
|
directory, because the Git client will run: |
|
|
|
git clone user@example.com:project.git |
|
| |
|
v |
|
ssh user@example.com "git-upload-pack 'project.git'" |
|
|
|
The exception is if a '~' is used, in which case |
|
we execute it without the leading '/'. |
|
|
|
ssh://user@example.com/~alice/project.git, |
|
| |
|
v |
|
ssh user@example.com "git-upload-pack '~alice/project.git'" |
|
|
|
A few things to remember here: |
|
|
|
- The "command name" is spelled with dash (e.g. git-upload-pack), but |
|
this can be overridden by the client; |
|
|
|
- The repository path is always quoted with single quotes. |
|
|
|
Fetching Data From a Server |
|
--------------------------- |
|
|
|
When one Git repository wants to get data that a second repository |
|
has, the first can 'fetch' from the second. This operation determines |
|
what data the server has that the client does not then streams that |
|
data down to the client in packfile format. |
|
|
|
|
|
Reference Discovery |
|
------------------- |
|
|
|
When the client initially connects the server will immediately respond |
|
with a listing of each reference it has (all branches and tags) along |
|
with the object name that each reference currently points to. |
|
|
|
$ echo -e -n "0039git-upload-pack /schacon/gitbook.git\0host=example.com\0" | |
|
nc -v example.com 9418 |
|
00887217a7c7e582c46cec22a130adf4b9d7d950fba0 HEAD\0multi_ack thin-pack |
|
side-band side-band-64k ofs-delta shallow no-progress include-tag |
|
00441d3fcd5ced445d1abc402225c0b8a1299641f497 refs/heads/integration |
|
003f7217a7c7e582c46cec22a130adf4b9d7d950fba0 refs/heads/master |
|
003cb88d2441cac0977faf98efc80305012112238d9d refs/tags/v0.9 |
|
003c525128480b96c89e6418b1e40909bf6c5b2d580f refs/tags/v1.0 |
|
003fe92df48743b7bc7d26bcaabfddde0a1e20cae47c refs/tags/v1.0^{} |
|
0000 |
|
|
|
Server SHOULD terminate each non-flush line using LF ("\n") terminator; |
|
client MUST NOT complain if there is no terminator. |
|
|
|
The returned response is a pkt-line stream describing each ref and |
|
its current value. The stream MUST be sorted by name according to |
|
the C locale ordering. |
|
|
|
If HEAD is a valid ref, HEAD MUST appear as the first advertised |
|
ref. If HEAD is not a valid ref, HEAD MUST NOT appear in the |
|
advertisement list at all, but other refs may still appear. |
|
|
|
The stream MUST include capability declarations behind a NUL on the |
|
first ref. The peeled value of a ref (that is "ref^{}") MUST be |
|
immediately after the ref itself, if presented. A conforming server |
|
MUST peel the ref if it's an annotated tag. |
|
|
|
---- |
|
advertised-refs = (no-refs / list-of-refs) |
|
flush-pkt |
|
|
|
no-refs = PKT-LINE(zero-id SP "capabilities^{}" |
|
NUL capability-list LF) |
|
|
|
list-of-refs = first-ref *other-ref |
|
first-ref = PKT-LINE(obj-id SP refname |
|
NUL capability-list LF) |
|
|
|
other-ref = PKT-LINE(other-tip / other-peeled) |
|
other-tip = obj-id SP refname LF |
|
other-peeled = obj-id SP refname "^{}" LF |
|
|
|
capability-list = capability *(SP capability) |
|
capability = 1*(LC_ALPHA / DIGIT / "-" / "_") |
|
LC_ALPHA = %x61-7A |
|
---- |
|
|
|
Server and client MUST use lowercase for obj-id, both MUST treat obj-id |
|
as case-insensitive. |
|
|
|
See protocol-capabilities.txt for a list of allowed server capabilities |
|
and descriptions. |
|
|
|
Packfile Negotiation |
|
-------------------- |
|
After reference and capabilities discovery, the client can decide to |
|
terminate the connection by sending a flush-pkt, telling the server it can |
|
now gracefully terminate, and disconnect, when it does not need any pack |
|
data. This can happen with the ls-remote command, and also can happen when |
|
the client already is up-to-date. |
|
|
|
Otherwise, it enters the negotiation phase, where the client and |
|
server determine what the minimal packfile necessary for transport is, |
|
by telling the server what objects it wants, its shallow objects |
|
(if any), and the maximum commit depth it wants (if any). The client |
|
will also send a list of the capabilities it wants to be in effect, |
|
out of what the server said it could do with the first 'want' line. |
|
|
|
---- |
|
upload-request = want-list |
|
*shallow-line |
|
*1depth-request |
|
flush-pkt |
|
|
|
want-list = first-want |
|
*additional-want |
|
|
|
shallow-line = PKT_LINE("shallow" SP obj-id) |
|
|
|
depth-request = PKT_LINE("deepen" SP depth) |
|
|
|
first-want = PKT-LINE("want" SP obj-id SP capability-list LF) |
|
additional-want = PKT-LINE("want" SP obj-id LF) |
|
|
|
depth = 1*DIGIT |
|
---- |
|
|
|
Clients MUST send all the obj-ids it wants from the reference |
|
discovery phase as 'want' lines. Clients MUST send at least one |
|
'want' command in the request body. Clients MUST NOT mention an |
|
obj-id in a 'want' command which did not appear in the response |
|
obtained through ref discovery. |
|
|
|
The client MUST write all obj-ids which it only has shallow copies |
|
of (meaning that it does not have the parents of a commit) as |
|
'shallow' lines so that the server is aware of the limitations of |
|
the client's history. Clients MUST NOT mention an obj-id which |
|
it does not know exists on the server. |
|
|
|
The client now sends the maximum commit history depth it wants for |
|
this transaction, which is the number of commits it wants from the |
|
tip of the history, if any, as a 'deepen' line. A depth of 0 is the |
|
same as not making a depth request. The client does not want to receive |
|
any commits beyond this depth, nor objects needed only to complete |
|
those commits. Commits whose parents are not received as a result are |
|
defined as shallow and marked as such in the server. This information |
|
is sent back to the client in the next step. |
|
|
|
Once all the 'want's and 'shallow's (and optional 'deepen') are |
|
transferred, clients MUST send a flush-pkt, to tell the server side |
|
that it is done sending the list. |
|
|
|
Otherwise, if the client sent a positive depth request, the server |
|
will determine which commits will and will not be shallow and |
|
send this information to the client. If the client did not request |
|
a positive depth, this step is skipped. |
|
|
|
---- |
|
shallow-update = *shallow-line |
|
*unshallow-line |
|
flush-pkt |
|
|
|
shallow-line = PKT-LINE("shallow" SP obj-id) |
|
|
|
unshallow-line = PKT-LINE("unshallow" SP obj-id) |
|
---- |
|
|
|
If the client has requested a positive depth, the server will compute |
|
the set of commits which are no deeper than the desired depth. The set |
|
of commits start at the client's wants. |
|
|
|
The server writes 'shallow' lines for each |
|
commit whose parents will not be sent as a result. The server writes |
|
an 'unshallow' line for each commit which the client has indicated is |
|
shallow, but is no longer shallow at the currently requested depth |
|
(that is, its parents will now be sent). The server MUST NOT mark |
|
as unshallow anything which the client has not indicated was shallow. |
|
|
|
Now the client will send a list of the obj-ids it has using 'have' |
|
lines, so the server can make a packfile that only contains the objects |
|
that the client needs. In multi_ack mode, the canonical implementation |
|
will send up to 32 of these at a time, then will send a flush-pkt. The |
|
canonical implementation will skip ahead and send the next 32 immediately, |
|
so that there is always a block of 32 "in-flight on the wire" at a time. |
|
|
|
---- |
|
upload-haves = have-list |
|
compute-end |
|
|
|
have-list = *have-line |
|
have-line = PKT-LINE("have" SP obj-id LF) |
|
compute-end = flush-pkt / PKT-LINE("done") |
|
---- |
|
|
|
If the server reads 'have' lines, it then will respond by ACKing any |
|
of the obj-ids the client said it had that the server also has. The |
|
server will ACK obj-ids differently depending on which ack mode is |
|
chosen by the client. |
|
|
|
In multi_ack mode: |
|
|
|
* the server will respond with 'ACK obj-id continue' for any common |
|
commits. |
|
|
|
* once the server has found an acceptable common base commit and is |
|
ready to make a packfile, it will blindly ACK all 'have' obj-ids |
|
back to the client. |
|
|
|
* the server will then send a 'NACK' and then wait for another response |
|
from the client - either a 'done' or another list of 'have' lines. |
|
|
|
In multi_ack_detailed mode: |
|
|
|
* the server will differentiate the ACKs where it is signaling |
|
that it is ready to send data with 'ACK obj-id ready' lines, and |
|
signals the identified common commits with 'ACK obj-id common' lines. |
|
|
|
Without either multi_ack or multi_ack_detailed: |
|
|
|
* upload-pack sends "ACK obj-id" on the first common object it finds. |
|
After that it says nothing until the client gives it a "done". |
|
|
|
* upload-pack sends "NAK" on a flush-pkt if no common object |
|
has been found yet. If one has been found, and thus an ACK |
|
was already sent, it's silent on the flush-pkt. |
|
|
|
After the client has gotten enough ACK responses that it can determine |
|
that the server has enough information to send an efficient packfile |
|
(in the canonical implementation, this is determined when it has received |
|
enough ACKs that it can color everything left in the --date-order queue |
|
as common with the server, or the --date-order queue is empty), or the |
|
client determines that it wants to give up (in the canonical implementation, |
|
this is determined when the client sends 256 'have' lines without getting |
|
any of them ACKed by the server - meaning there is nothing in common and |
|
the server should just send all of its objects), then the client will send |
|
a 'done' command. The 'done' command signals to the server that the client |
|
is ready to receive its packfile data. |
|
|
|
However, the 256 limit *only* turns on in the canonical client |
|
implementation if we have received at least one "ACK %s continue" |
|
during a prior round. This helps to ensure that at least one common |
|
ancestor is found before we give up entirely. |
|
|
|
Once the 'done' line is read from the client, the server will either |
|
send a final 'ACK obj-id' or it will send a 'NAK'. The server only sends |
|
ACK after 'done' if there is at least one common base and multi_ack or |
|
multi_ack_detailed is enabled. The server always sends NAK after 'done' |
|
if there is no common base found. |
|
|
|
Then the server will start sending its packfile data. |
|
|
|
---- |
|
server-response = *ack_multi ack / nak |
|
ack_multi = PKT-LINE("ACK" SP obj-id ack_status LF) |
|
ack_status = "continue" / "common" / "ready" |
|
ack = PKT-LINE("ACK SP obj-id LF) |
|
nak = PKT-LINE("NAK" LF) |
|
---- |
|
|
|
A simple clone may look like this (with no 'have' lines): |
|
|
|
---- |
|
C: 0054want 74730d410fcb6603ace96f1dc55ea6196122532d multi_ack \ |
|
side-band-64k ofs-delta\n |
|
C: 0032want 7d1665144a3a975c05f1f43902ddaf084e784dbe\n |
|
C: 0032want 5a3f6be755bbb7deae50065988cbfa1ffa9ab68a\n |
|
C: 0032want 7e47fe2bd8d01d481f44d7af0531bd93d3b21c01\n |
|
C: 0032want 74730d410fcb6603ace96f1dc55ea6196122532d\n |
|
C: 0000 |
|
C: 0009done\n |
|
|
|
S: 0008NAK\n |
|
S: [PACKFILE] |
|
---- |
|
|
|
An incremental update (fetch) response might look like this: |
|
|
|
---- |
|
C: 0054want 74730d410fcb6603ace96f1dc55ea6196122532d multi_ack \ |
|
side-band-64k ofs-delta\n |
|
C: 0032want 7d1665144a3a975c05f1f43902ddaf084e784dbe\n |
|
C: 0032want 5a3f6be755bbb7deae50065988cbfa1ffa9ab68a\n |
|
C: 0000 |
|
C: 0032have 7e47fe2bd8d01d481f44d7af0531bd93d3b21c01\n |
|
C: [30 more have lines] |
|
C: 0032have 74730d410fcb6603ace96f1dc55ea6196122532d\n |
|
C: 0000 |
|
|
|
S: 003aACK 7e47fe2bd8d01d481f44d7af0531bd93d3b21c01 continue\n |
|
S: 003aACK 74730d410fcb6603ace96f1dc55ea6196122532d continue\n |
|
S: 0008NAK\n |
|
|
|
C: 0009done\n |
|
|
|
S: 0031ACK 74730d410fcb6603ace96f1dc55ea6196122532d\n |
|
S: [PACKFILE] |
|
---- |
|
|
|
|
|
Packfile Data |
|
------------- |
|
|
|
Now that the client and server have finished negotiation about what |
|
the minimal amount of data that needs to be sent to the client is, the server |
|
will construct and send the required data in packfile format. |
|
|
|
See pack-format.txt for what the packfile itself actually looks like. |
|
|
|
If 'side-band' or 'side-band-64k' capabilities have been specified by |
|
the client, the server will send the packfile data multiplexed. |
|
|
|
Each packet starting with the packet-line length of the amount of data |
|
that follows, followed by a single byte specifying the sideband the |
|
following data is coming in on. |
|
|
|
In 'side-band' mode, it will send up to 999 data bytes plus 1 control |
|
code, for a total of up to 1000 bytes in a pkt-line. In 'side-band-64k' |
|
mode it will send up to 65519 data bytes plus 1 control code, for a |
|
total of up to 65520 bytes in a pkt-line. |
|
|
|
The sideband byte will be a '1', '2' or a '3'. Sideband '1' will contain |
|
packfile data, sideband '2' will be used for progress information that the |
|
client will generally print to stderr and sideband '3' is used for error |
|
information. |
|
|
|
If no 'side-band' capability was specified, the server will stream the |
|
entire packfile without multiplexing. |
|
|
|
|
|
Pushing Data To a Server |
|
------------------------ |
|
|
|
Pushing data to a server will invoke the 'receive-pack' process on the |
|
server, which will allow the client to tell it which references it should |
|
update and then send all the data the server will need for those new |
|
references to be complete. Once all the data is received and validated, |
|
the server will then update its references to what the client specified. |
|
|
|
Authentication |
|
-------------- |
|
|
|
The protocol itself contains no authentication mechanisms. That is to be |
|
handled by the transport, such as SSH, before the 'receive-pack' process is |
|
invoked. If 'receive-pack' is configured over the Git transport, those |
|
repositories will be writable by anyone who can access that port (9418) as |
|
that transport is unauthenticated. |
|
|
|
Reference Discovery |
|
------------------- |
|
|
|
The reference discovery phase is done nearly the same way as it is in the |
|
fetching protocol. Each reference obj-id and name on the server is sent |
|
in packet-line format to the client, followed by a flush-pkt. The only |
|
real difference is that the capability listing is different - the only |
|
possible values are 'report-status', 'delete-refs' and 'ofs-delta'. |
|
|
|
Reference Update Request and Packfile Transfer |
|
---------------------------------------------- |
|
|
|
Once the client knows what references the server is at, it can send a |
|
list of reference update requests. For each reference on the server |
|
that it wants to update, it sends a line listing the obj-id currently on |
|
the server, the obj-id the client would like to update it to and the name |
|
of the reference. |
|
|
|
This list is followed by a flush-pkt and then the packfile that should |
|
contain all the objects that the server will need to complete the new |
|
references. |
|
|
|
---- |
|
update-request = command-list [pack-file] |
|
|
|
command-list = PKT-LINE(command NUL capability-list LF) |
|
*PKT-LINE(command LF) |
|
flush-pkt |
|
|
|
command = create / delete / update |
|
create = zero-id SP new-id SP name |
|
delete = old-id SP zero-id SP name |
|
update = old-id SP new-id SP name |
|
|
|
old-id = obj-id |
|
new-id = obj-id |
|
|
|
pack-file = "PACK" 28*(OCTET) |
|
---- |
|
|
|
If the receiving end does not support delete-refs, the sending end MUST |
|
NOT ask for delete command. |
|
|
|
The pack-file MUST NOT be sent if the only command used is 'delete'. |
|
|
|
A pack-file MUST be sent if either create or update command is used, |
|
even if the server already has all the necessary objects. In this |
|
case the client MUST send an empty pack-file. The only time this |
|
is likely to happen is if the client is creating |
|
a new branch or a tag that points to an existing obj-id. |
|
|
|
The server will receive the packfile, unpack it, then validate each |
|
reference that is being updated that it hasn't changed while the request |
|
was being processed (the obj-id is still the same as the old-id), and |
|
it will run any update hooks to make sure that the update is acceptable. |
|
If all of that is fine, the server will then update the references. |
|
|
|
Report Status |
|
------------- |
|
|
|
After receiving the pack data from the sender, the receiver sends a |
|
report if 'report-status' capability is in effect. |
|
It is a short listing of what happened in that update. It will first |
|
list the status of the packfile unpacking as either 'unpack ok' or |
|
'unpack [error]'. Then it will list the status for each of the references |
|
that it tried to update. Each line is either 'ok [refname]' if the |
|
update was successful, or 'ng [refname] [error]' if the update was not. |
|
|
|
---- |
|
report-status = unpack-status |
|
1*(command-status) |
|
flush-pkt |
|
|
|
unpack-status = PKT-LINE("unpack" SP unpack-result LF) |
|
unpack-result = "ok" / error-msg |
|
|
|
command-status = command-ok / command-fail |
|
command-ok = PKT-LINE("ok" SP refname LF) |
|
command-fail = PKT-LINE("ng" SP refname SP error-msg LF) |
|
|
|
error-msg = 1*(OCTECT) ; where not "ok" |
|
---- |
|
|
|
Updates can be unsuccessful for a number of reasons. The reference can have |
|
changed since the reference discovery phase was originally sent, meaning |
|
someone pushed in the meantime. The reference being pushed could be a |
|
non-fast-forward reference and the update hooks or configuration could be |
|
set to not allow that, etc. Also, some references can be updated while others |
|
can be rejected. |
|
|
|
An example client/server communication might look like this: |
|
|
|
---- |
|
S: 007c74730d410fcb6603ace96f1dc55ea6196122532d refs/heads/local\0report-status delete-refs ofs-delta\n |
|
S: 003e7d1665144a3a975c05f1f43902ddaf084e784dbe refs/heads/debug\n |
|
S: 003f74730d410fcb6603ace96f1dc55ea6196122532d refs/heads/master\n |
|
S: 003f74730d410fcb6603ace96f1dc55ea6196122532d refs/heads/team\n |
|
S: 0000 |
|
|
|
C: 003e7d1665144a3a975c05f1f43902ddaf084e784dbe 74730d410fcb6603ace96f1dc55ea6196122532d refs/heads/debug\n |
|
C: 003e74730d410fcb6603ace96f1dc55ea6196122532d 5a3f6be755bbb7deae50065988cbfa1ffa9ab68a refs/heads/master\n |
|
C: 0000 |
|
C: [PACKDATA] |
|
|
|
S: 000eunpack ok\n |
|
S: 0018ok refs/heads/debug\n |
|
S: 002ang refs/heads/master non-fast-forward\n |
|
----
|
|
|